I think a Baja is your best option I used to have one and raced it at KORT down in Cornwall they can be quite competitive once fitted with a decent steering servo such as a hitec or rhino available from rossmods. A local racer used to regularly make A finals at BRCA nationals but has now moved to a hormann a very competitive car winning BRCA nationals at the moment.
For any help and advice check out
http://team5hite.forumup.com/index.php?mforum=team5hite great bunch of guys that will help you out.
Callum